The present invention relates generally to cartridge type heaters, and more particularly to a cartridge heater having novel means for releasably retaining the cartridge heater within an oversize cavity into which the heater may be inserted.
It is a common practice in many types of machines, such as plastic molding machines and the like, to provide heater bore-holes or cavities in selected positions in a mold or die support housing or in a heater support header. Cartridge type heaters are inserted into the bore-holes or cavities to facilitate heating of the associated molds to desired temperatures during operation. The cartridge heaters conventionally include cylindrical metallic tubular sheaths which receive inner heating cores having electrical wire resistance elements adapted for connection to an electrical power supply. The wire resistance element is generally helically coiled on an insulating core, and a mineral insulating material having low electrical conductivity and high thermal conductivity, such as magnesium oxide, is compacted into the annular space between the coiled core and the outer sheath.
The heater receiving bores or cavities and the cartridge heater sheaths are generally cylindrical and sized to provide a snug fit for optimal heat transfer while enabling the cartridge heaters to be readily inserted into and removed from the bore-holes or cavities. It is important that the cartridge heaters be retained within the bore-holes or cavities so as to resist displacement from the cavities when subjected to vibration or other motion during operation of the associated machine. Unintentional displacement from the heater cavity may lead to overheating and burnout, and thus premature failure of the cartridge heater. This may present a significant problem when the cross-sectional area of the heater receiving bore or cavity is oversize, which may result if the cavity has a relatively broad tolerance range.
A prior technique for addressing the aforedescribed problem has been to secure an external washer or bracket to the cartridge heater which enables the heater to be physically attached to the device to be heated. This technique has significant drawbacks such as increased cost and limited applicability where space limitations prevent its use.
Another prior technique for addressing the problem of oversize heater cartridge bores utilizes metallic sleeves which have inside diameters to closely receive associated cartridge sheaths, and have outside diameters to closely fit within the larger diameter bore. See, for example, U.S. Pat. No. 3,937,923. A disadvantage of this technique is the requirement for an inventory of sleeves needed to meet varying diameter cartridge heater bores.
Still another prior technique for addressing this problem is to form tapered heater cartridge bore holes in the header flange, and to place the cartridge heater into a heat conducting compressible tapered bushing which is then driven into the tapered bore hole until the bushing fits snugly into the bore hole and lightly holds the cartridge heater in place. See, for example, U.S. Pat. No. 3,412,231.